Treacherous person finished protecting Queen (7)
I believe the answer is:
serpent
'treacherous person' is the definition.
'finished protecting queen' is the wordplay.
'finished' becomes 'spent' (I've seen this before).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'spent' enclosing 'er' is 'SERPENT'.
(Other definitions for serpent that I've seen before include "Present for a snake" , "Devil of a snake" , "Malicious person" , "exhibit at zoo" , "the first lady found beguiling" .)